Most high purity silicon used in the production of solar cells is presently produced using a chlorine chemical technique known as the so-called Simens method. However, although silicon of sufficiently high purity used for producing semiconductor elements can be obtained based on this Simens method, it is problematic in that production costs are high, silicon chloride waste regarded as poisonous gases are generated in large quantities, substantial resources are required in order to construct production facilities, etc. Accordingly, regarding the production of high purity silicon used in producing solar cells for which increased demand is expected in the future, the supply is limited by the Simens method.
On the other hand, regarding high purity silicon used in the production of solar cells, as sufficiently high purity for producing semiconductor elements is not required, a process for producing cheap high purity silicon in large quantities has been developed using metallurgical techniques such as vacuum melting, solidification refining, etc.
The production process of high purity silicon based on these metallurgical techniques involves a process of refining impure elements by combining metallurgical sub-processes using differences in physical behaviour between impure elements and silicon, wherein the application of vacuum melting utilizing a sub-process that removes impure elements with higher vapor pressures than Si (silicon) represented by P (phosphorous) has been specifically considered. Hereinafter, the removal of P via this vacuum melting is referred to as “P removal”; moreover, during this P removal, impure elements other than P with higher vapor pressures than Si are simultaneously removed.
The silicon refining device used for this vacuum melting is basically of a configuration in which a crucible for housing a metal silicon material and a heating device such as a heater are disposed in a vessel capable of reducing the pressure (a decompression vessel) having a vacuum pump. Further, a metal silicon material containing P higher than several tens of ppm is filled into said crucible and this metal silicon material is heated and melted into an inactive gas under decompression, enabling P removal by maintaining the obtained silicon molten metal for a constant time under decompression and at a temperature no lower than the melting point. The P concentration in Si decreases with time as P with a higher vapor pressure than Si is selectively vaporized by this P removal operation.
As a silicon refining device that has been used thus far in the implementation of vacuum melting, conventional arts such as described in Patent Document 1, Non-patent Document 1, Non-patent Document 2, Non-patent Document 3, Non-patent Document 4, etc. are known. As the silicon refining device used for these conventional arts has a basic structure in which a crucible and a general heating device are disposed in a decompression vessel, the silicon refining device has a cheap device configuration; however, the silicon refining device has a low P removal rate, in other words, it is problematic in that the silicon refining device is impractical due to its low productivity. Moreover, regarding some of the abovementioned conventional arts, it has been reported that the P concentration in the metal silicon material cannot be refined to approximately 0.05 ppm or less, as is required for high purity silicon used in the production of solar cells, making it a quality problem as well.
On the other hand, conventional arts as described in Non-patent Document 5, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, Patent Document 8 are also known as practical silicon refining methods with high P removal rates and high productivity. However, as these conventional arts are based on electron beam melting, the facilities and facility costs become enormous, creating a practical problem due to facility costs. Particularly, as disclosed in Patent Document 2 and Patent Document 7, based on a method of using electron beams as a means of heating and melting, it is necessary to set a plurality of crucibles in a vacuum vessel, which is problematic in that greater facility costs are required.
Against such a background, a silicon refining device that can achieve a sufficient P removal rate using a comparatively simple and cheap device configuration has been suggested, the silicon refining device comprising: a crucible in a decompression vessel provided with a vacuum pump; a heating device in the decompression vessel; and an impurity condenser provided with a cooling system and a raising and lowering device configured to condense and remove impurities evaporated from a silicon molten metal during heating of the crucible, which is arranged on a silicon molten metal surface in said crucible or at a location from which one or both crucible opening portions can be seen (Patent Document 9).
Moreover, in Patent Document 10, a silicon refining device is proposed, wherein, in the silicon refining device having a crucible and a heating device in a decompression vessel provided with a vacuum pump, for example, as illustrated in FIG. 8 and FIG. 9, molten metal surface thermal insulation member 1 is provided in the upper portion of the crucible, said molten metal surface thermal insulation member 1 comprising: support member 3 having support part 3a formed in a ring shape by a graphite flat plate with the center thereof open and cylindrical part 3b constructed on the center opening edge of this support part 3a of a certain height; insulation material 4 made of a fiber such as a carbon-made felt provided on the upper face of support part 3a of this support member 3; and exhaust opening 2 formed by cylindrical part 3b of said support member 3, with a smaller opening area than the silicon molten metal surface area in said crucible; wherein, the silicon molten metal surface is moisturized by this molten metal surface thermal insulation member 1 to be deposited onto the surface of the silicon molten metal for preventing SiC particles from obstructing the vaporization of impurities and further, in Patent Document 11, in lieu of such a molten metal surface thermal insulation member, a silicon refining device is proposed, the silicon refining device covering the upper end opening of a crucible at the upper portion of the crucible and having an exhaust opening with a smaller opening area than the silicon molten metal surface area, within the crucible, comprising a ring-shaped moisturizing cap that can be exchanged while heating the crucible.